This is a national stage application of PCT/F198/00362, filed Apr. 24, 1998.
The present invention relates to a method and apparatus for coating a moving board or paper web by the film transfer coating method and for coating the web twice whereby the film transfer coater of the invention is used in the first or the second coating step.
Paper or board can be coated with one or more coating layers in order to improve its printability. The more coating layers are applied, the higher the attained paper quality, simultaneously enabling the use of coating agents having different properties. However, the use of two coating layers entails a significant increase in production costs because the coating is usually carried out in separate steps and the applied coating layer has been dried prior to the application of the next layer.
The coating can be performed either directly onto the surface of the paper by means of e.g. a blade or a rod coater or by means of a film transfer coater whereby film which has been premetered onto a film roll is transferred onto the paper surface in a roll nip. It is typical of the blade and rod coating methods that the doctor blade fills the roughness volume of the coating underlayer and evens out the surface whereby the thickness of the coat varies in accordance with the roughness volume variations of the coating underlayer. A smooth coat results having an uneven brightness coverage, and the uniform absorption properties of the coat are difficult to control.
In film transfer coating, a coat of an essentially more uniform thickness is obtained rendering it easy to control the absorption properties, but sufficient smoothness properties pose a problem particularly in the case of thicker papers and boards. The coating also provides quite an even coverage whereby, for example, the brightness of a coating underlayer of low brightness can be significantly improved by means of this coating method. Furthermore, as no doctor blade trailing along the coating underlayer is involved in film transfer coating, the method offers excellent runnability with respect to coating breaks. A problem hampering film transfer coating, however, is posed by the fuming of the coating and the so called orange peel pattern which is due to splitting of the surface of the coating on the web detached from the film transfer roll and the film forming the coat on the roll surface, the orange peel pattern being observable as small craters in the coating. Such fuming and the formation of an orange peel pattern restrict to some extent the maximum coating speed.
Two separate ways of exploiting the special properties of the blade and film transfer coating methods have been sought.
Film transfer coating is widely used in precoating and in front coating. When used as precoat it improves the coating result due to the total coat weight alone, and the inclusion of even just one blade coat is a guarantee for sufficient smoothness. The precoat is usually dried before the following surface coat which is usually applied by means of a blade coater. From U.S. Pat. No. 2,937,955, it is also known to perform blade coating directly onto the half-wet film precoating. A prerequisite for this method is that the precoat is sufficiently set to withstand surface coating by a blade. The setting can be enhanced e.g. by partial drying of the precoat if the coat has not undergone sufficient dehydration due to filtration caused by pressure penetration in the coating nip and water absorption in the free gap between the pre- and surface coaters.
The delay and setting time between the film precoating and the blade surface coating can be extended e.g. by increasing the length of the travel of the web between the coaters, whereby a sufficiently set and dried precoat is achieved during the extended absorption time into the web for the next blade coating step. U.S. Pat. No. 5,340,611 describes a method where the amount of film coating is kept so small that the doctor blade of the blade coater does not remove the precoat by doctoring. In this manner, however, the great amount of applied coating allowed by the film transfer coater cannot be exploited, and the resulting thickness of the first coating layer is very small. In the second step, it is also possible to use a rod instead of a blade.
In pilot-scale test runs for coating paperboard, the 10 g/m2 precoat could not be made to set sufficiently by the method even if the delay time between the coating events was extended, in order for the next blade coating step not to partly remove the already applied precoat by doctoring. A further problem was that when the web speed was determined by the film coating nip and when the web speed was reduced by the following blade coating event, a tendency appeared for a bag or a loose section to be formed in front of the blade resulting in web breaks. Thus, in this case the method did not work. The probability of web breaks is further increased because the blade coating which strains the web to a much greater extent than film transfer coating is performed while the web is damp, the strength of a damp web naturally being inferior to the strength of a dry web.
A second alternative for exploiting the special properties of film and blade coats is described in the applicant""s Finnish Patent Application No. 941803 wherein the coating unit is constructed such that either film transfer coating or blade coating can be used as alternative methods depending on what is required of the final product and/or on considerations relating to runnability, whereby, however, the same coat drying apparatus is used for both methods. This apparatus can not be used to exploit the benefits of two-layer coating wherefore it chiefly increases the flexibility of production but does not improve the quality of the final product as compared to corresponding single-layer coating methods.
Particularly in the case of thick papers and boards the need exists to combine the benefits provided by film transfer coating and blade coating. Due to the space required as well as the investment costs incurred, however, intermediate drying of the precoat is not always possible. In this case the only alternative available is the above-described wet-on-wet coating, but current methods will not achieve sufficient total coat weights to allow the best possible exploitation of the good coverage obtained by film transfer coating and the good smoothness properties achieved by blade coating.
The film transfer coating method can also be used to provide the surface coat whereby the layer produced by the film transfer coating method and applied onto the first layer which has been evened out by means of a blade or rod is smooth.
The present invention is particularly well suited for wet-on-wet coating whereby the second coating layer is applied onto an at least partly wet first coating layer.
The present invention aims at achieving a film transfer coating method by which the coating event can be controlled more reliably than has been the case, thus obtaining improved runnability and increased coating speed of the film transfer coater.
The invention is based on taking the web being coated through the film transfer nip supported by a belt, and the angle of the belt over the applicator roll is controlled such that the contact distance between the web and the roll can be altered.
Further in the double coating method according to the invention the web is taken through two successive coating steps supported by a belt and one coating step is performed by means of a film transfer coater and the other e.g. by means of a short-dwell coater.
The invention offers considerable benefits.
The controllability of the roll applicator station can be essentially improved by means of the belt because by varying the incoming angle and the angle of departure of the belt it is possible to affect the introduction of the web being coated into the nip, its detachment from the surface of the applicator roll, and the wetting time and pressure. By extending the wetting time, i,e the delay time of the film transfer event, a better absorption of the coating into the web is achieved, whereby a thicker first coating layer can be made to adhere to the surface of the web. Particularly in double coating this has a significant effect because, due to the better absorption, a sufficient amount of coating can be made to remain on the web surface even after blade coating. If, then, the blade coating step precedes the film transfer coating, the belt provides improved coating smoothness because it can be used to control the angle of detachment of the web from the applicator roll such that the formation of, e.g. an orange peel pattern is minimal. The use of a belt and the belt angle control before the web enters the nip and when it leaves the nip serve to provide further improved runnability of the web, and controlled web detachment reduces the formation of the orange peel pattern. Naturally, a rod can be used as the doctoring means instead of a blade.
In applicator roll application the belt replaces the soft roll. The belt is easier to replace once it is worn and it is considerably less costly than a roll. Thus, the belt can be replaced after short periods of use. Good runnability is provided in double coating because in the first step, the wetted web is supported by a belt whereby the web itself is subjected to less strain. A belt-supported web does not form bags as easily as a web which lacks support. In the precoating step, an inexpensive coating mix can be used to even out the surface of the paper, and blade coating is performed using a more costly finely divided coating mix, whereby good printability and coverage properties are obtained.
Belt support during the entire film transfer process provides additional possibilities of coating surface sizing even the lightest paper grades. The support can be utilized to better control detachment of the web from the film transfer roll without the web flapping between the roll surface and the supporting belt. Uncontrolled detachment exposes the web to mechanical strain which may lead to marking of the treated surface or even break the web. The extended contact distance after the roll nip reduces fuming of the coat with the increasing distance between the nip and the point of detachment and the decreasing angle of detachment because the separation force is then reduced at the splitting point of the film.
By changing the distance of the contact point between the belt-supported web and the film transfer coater from the roll nip it is possible to increase the amount of coating mix transferred onto the web. When the web touches the coating film on the roll surface, water contained in the coating begins to be transferred to the web due to capillary and pressure penetration. A layer of packed pigment particles is formed on the web surface at the coating/web interface, which reduces the flow speed of the liquid phase and the solid phase onto the web. The contact distance can be varied in order to provide or to maintain the desired contact time at a greater speed or a greater coat weight or when the driving parameters are altered in some other way. Belt stretch can be used to control the application pressure but the pressure pulse caused by the belt is always smaller than that of the roll nip.
The increased contact distance makes a greater run speed and a greater coat weight possible with one application event without runnability problems such as fuming of the coat from the nip of formation of patterns on the coated surface. Due to the increased contact distance, an improved coat coverage is also obtained and penetration of the coating into the web is reduced and the coating is kept on the web surface.